Many attempts have been made to provide a gas valve that can be converted for use with either Natural Gas or Liquid Propane Gas. Several of these gas valve designs require replacement or readjustment of a biasing spring during conversion, as disclosed in Reinhardt et al, U.S. Pat. No. 5,988,204. Another such design is disclosed in Dietiker, U.S. Pat. No. 5,413,141. This gas valve design provides for first or second stage operation in response to a furnace control signal that switches the gas valve""s pressure regulation from a first servo-regulator diaphragm to a second servo-regulator diaphragm. The gas valve further includes a selection means for converting both first and second servo-regulator diaphragm settings from a Natural Gas setting to a LP Gas setting. However, certain applications which demand more than two stages of gas valve operation require a modulating gas valve, as disclosed in Dietiker, U.S. Pat. No. 5,215,115. This gas valve is capable of modulating the outlet pressure in response to an electronic signal, but does not allow for conversion from Natural Gas pressure to LP Gas pressure. Natural Gas is typically supplied to a furnace at pressures up to 3.5 inches of water column, while LP Gas is supplied at pressures up to 10 inches of water column. Although modulating gas valves are available which can be converted from Natural Gas to LP Gas by readjustment of regulator settings, an increasing need exists for an electronically modulated gas valve that can be converted from Natural Gas to LP Gas operation without any readjustment.
The present invention provides for the use of either Natural Gas or LP Gas in an electronically modulating gas valve for a gas-fired heating apparatus, while simplifying the conversion from Natural Gas to LP Gas pressure settings. The pressure regulation means of the present invention comprises a modulating solenoid for simultaneously displacing two servo-regulator diaphragms, one for controlling a Natural Gas outlet pressure and the other for controlling a LP Gas outlet pressure. A switch is used to select operation with either Natural Gas or LP Gas, wherein the switch activates a gas selection solenoid for controlling gas flow through the Natural Gas servo-regulator and the LP Gas servo-regulator. Both servo-regulators are factory-adjusted for a maximum Natural Gas pressure setting and a maximum LP Gas pressure setting. Thus, the electronically modulating gas valve is capable of varying the supply of either Natural Gas or LP Gas up to their respective maximum outlet pressures, and is convertible from Natural Gas to LP Gas without requiring any adjustments.